1. Field of the Invention
The present invention relates to a discharge-in-magnetic-field type, (more specifically, crossed-field discharge type) ion generating apparatus use as an ion source in a mass spectrograph, and in surface analyzing equipment and the like.
2. Description of the Related Art
Generally, the ion generating apparatus has been used as a device of implanting ions into semiconductor and an ion source for various kinds of accelerators. In addition to such applications, the ion generating apparatus has been utilized to ionize atoms and molecules for analysis by a gas analyzer mounted to a vacuum equipment, and for surface analyzing equipment for solid.
Many ion generating apparatuses utilize a discharge either a DC discharge or an AC discharge. A characteristic of the device utilizing the DC discharge is stability. In order to generate a DC discharge, a positive pole and a negative pole are needed. With temperature of the negative pole as the criterion, the DC discharge may be classified into hot cathode and a cold cathode type. The ion generating apparatus, having no high temperature part whose DC discharge is the cold cathode type, is featured by a minimized wear of the negative pole and a longer life of the apparatus.
"Penning discharge type" is found as a typical example of the cold cathode DC discharge type ion generating apparatus. The Penning discharge the discharge occurs in gas whose pressure has been decreased and is a "crossed-field discharge", wherein the discharge is performed between the positive pole of positive potential, having a hollow part, which is put in a magnetic field and the negative pole of negative potential which is disposed to cover 2 opening parts of the afore-mentioned hollow part. Where the discharge substantially occurs, an electric field and the magnetic field are perpendicular jointed to each other, electrons are confined in the electric and magnetic fields, a group of electrons is formed, and collision between molecules of the gas and the electrons cause the molecules to be ionized. The ions are generated in the space where a group of the electrons exists, i.e. in the discharge space, and are injected through a through hole made in a position of the negative pole corresponding to the center line of the hollow part of the positive pole. Thus, the Penning discharge type ion generating apparatus supplies the ions to the outside.
If the through hole is made in the negative pole, the penning discharge may be unstabilized. In that case, a state of instability of the Penning discharge can be avoided, if the electric potential of one negative pole at which the ion injecting hole is located can be made lower enough compared with the other negative pole in which the injecting hoe is not found, or if the electric potential of substance placed close to and just outside of the ion injecting hole lower than that of the negative pole of the ion generating apparatus, so any specified problem is avoided. However, the Penning discharge type ion generating apparatus faces still the following problem:
The given solid (object) is placed at the position, opposite to the ion injecting hole, of the negative pole in which the ion injecting hole is not found and surface of the solid is irradiated by the ions occurring during the discharge, thereby sputtering substances from the surface of the solid. A method of ionizing the neutral particles, emitted by the sputtering, by the discharge is utilized as a sputtering type ion source. An official gazette of Japanese patent unexamined application No. SHO (59)-121746 (hereinafter referred to as document 1) discloses a possibility of utilizing also such a method for means of analyzing the surface of a solid.
It is possible to realize the afore-mentioned method using a Penning discharge, but the requirement for that case is, as mentioned above, that the electric potential of the negative pole at which the solid to be sputtered is located shall be made higher than that of the negative pole at which the through hole is located. But under such a method, energy of the incident irons into the surface of the solid to be sputtered cannot reach the level being required for increasing the sputtering ratio as much as possible. Therefore, the device using the Penning discharge cannot increase value of the output ionic current in the case of the sputtering type ion source (equipment), while it cannot make analytical sensitivity higher in the case of the surface analyzer. In order to solve such conventional problems as mentioned above, the document 1 discloses a crossed-field triode discharge, wherein a control electrode is added to a group of electrodes for the Penning discharge.
FIG. 12 is a longitudinal sectional view of the principal part of the surface analyzer referred to in the document 1, rewritten for the purpose of emphasizing its gist only without a change of its technical content.
FIG. 13 is a power connection diagram illustrating a relationship of the electric potentials among the electrodes. For a purpose of simplicity only, all the drawings in the specification employ the same reference characters common to the corresponding parts.
Thus, the ion generating apparatus in use for the surface analyzer of FIG. 12 generates the ions from the surface substance of the sample. After some parts of the generated ions pass through the ion injecting hole 8 made in the 2nd negative pole 7, an incidence of them into an ion mass separator 9 is made and each current value of the ions whose mass has been separated is measured by an ion current measuring device 10. The specified method is employed to change the ions' mass being separated and the ions' mass and the ion's current are set against one another, thereby performing an ions' mass spectrometry, by which a surface analysis of the sample 6a is in turn executed.
FIG. 13 specifies no grounding point in an illustrated manner. The variants of a mode of setting the grounding points are considered to be subject to its relationship with the ion mass separator 9 and, whichever variants are selected, any influence of them upon an actuation of the ion generating apparatus by itself which uses the crossed-field triode discharge may not take place. With the ion generating apparatus of the afore-mentioned construction, an electric potential of the discharge space is determined mainly by the electric potentials of the control electrode 5 and the positive pole 3, if both the electric potentials of the two negative poles 6 and 7 are sufficiently lower than that of the control electrode 5, so that there is no relation between the discharge space and the electric potentials of the two negative poles 6 and 7. A variation in kinetic energy of the ions incident upon the negative poles 6 and 7 is subject to a difference between the electric potential of the discharge space and the electric potentials of the negative poles 6 and 7. For this reason, if the kinetic energy of the ions incident upon the negative poles 6 and 7 is desired to settle within somewhat larger value, it may be optionally established so that it becomes possible to set the sputtering ratio of the substance in the negative pole to higher level, so the afore-mentioned problem that the sputtering ratio cannot be increased, when the Penning discharge is used, may be solved.
Nevertheless, the ion generating apparatus using the crossed-field triode discharge still faces such a problem of impurity irons. A description of the problem is made in conjunction with FIG. 12 in the context of an example of an ion generating apparatus for the surface analyzer.
The ions which have been generated in the discharge space irradiate the control electrode 5 and the 2nd negative pole 7 in addition to the sample 6a, whereby each surface substance of the control electrode 5 and the 2nd negative pole 7 is sputtered to be emitted into the discharge space, and, similarly to the surface substance of the sample, the former substance is ionized, the resulting ions are injected from the ion injecting hole 8 and their mass is to be analyzed, such a result of the mass analysis of the ions working a background of the surface analysis of the sample. Since the background makes ordinarily a ratio between a signal of the surface analysis of the sample and a noise (SN ratio) smaller, a sensitivity of the analysis may be unavoidably limited to lower level. Furthermore, because the ions of the surface substance of the control electrode 5 and the 2nd negative pole 7 are not ones of the surface substance of the sample, the former ions are the impurity ions. As a result, the impurity ions are detrimental to the surface analysis of the sample. A reduction in amount of the impurity ions and an increase in the SN ratio have been demanded. Similarly, since there occurs also such a problem as a mixture of the impurity ions into an output ion current, if the crossed-field triode type ion generating apparatus is used as the sputtering type ion source equipment, a need exists to reduce an amount of the impurity ion.
Thus, if the conventional Penning discharge type ion generating apparatus, having the solid substance to be ionized arranged on the negative pole, is used as a sputtering type ion source, a sputtering ratio of the solid substance to be ionized cannot be enlarged, so the value of output ion-beam current cannot be increased. If the conventional apparatus is used as the surface analyzer with the sample undergoing surface analysis, being disposed on the negative pole, also for the afore-mentioned reason, the analysis sensitivity is limited.
Although the afore-mentioned problems incurred by the Penning discharge type ion generating apparatus are solved by the crossed-field triode discharge type ion generating apparatus, if the crossed-field triode is used as the sputtering type ion source, the impurity ions which are generated from the surface substance of the control electrode and the 2nd negative pole are mixed in the output ion-beam, while if it is used as a surface analyzer, a limit of the SN ratio of the analysis to smaller value decreases in turn the analysis sensitivity.